Identifying Bacteria in a Petri Dish

Culturing and identifying bacteria are important in controlling antibiotic resistance. The number of bacterial species resistant to antibiotics is growing, and this is partially a result of doctors over-prescribing antibiotics. Unless a patient is in a life-threatening situation, culturing the bacterium prior to antibiotic treatment is safer for the patient's health and the community.

Things You'll Need

• Light microscope

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Instructions

• 1

  Wash your hands. It's important to wash your hands both before and after handling bacteria cultures. Unwashed hands will contaminate the sample with other bacteria species. Washing your hands after handling the cultures ensures that you won't spread them to others.

• 2

  Gram stain a sample on a microscope slide. Gram staining identifies the cell wall structure of the bacteria, and it uses a purple dye called crystal violet, which is dropped on the microbe sample. The cell wall of the bacteria, called the peptidoglycan, absorbs the purple dye. A dark purple stain under the microscope identifies the bacteria as a gram-positive species, meaning there is a higher peptidoglycan content than in gram-negative bacteria. Gram-positive bacteria have more virulence factors, but the higher content of peptidoglycan makes them more susceptible to antibiotics. A pinkish color identifies the bacteria as gram-negative, meaning its cell walls are thinner and less complex.

• 3

  Identify the shape of the bacteria. Round, circular shapes are named using the term "coccus," while rod-shaped bacteria are termed "bacillus." Spiral-shaped bacteria are more difficult to identify, since they look similar to bacillus. Spirals are termed "spirilli."

• 4

  Identify the type of grouping. Bacillus are normally found in pairs or long chains, giving them the prefix "diplo." Coccus are normally found in clusters, and they are termed "staphylococcus." Long chains of coccus are termed "streptococcus."

• 5

  Perform an antibiotic resistance test. Some species of bacteria are resistant to certain antibiotics and susceptible to others. Using several different antibiotics, such as penicillin, cephalosporins and fluoroquinolones, test to see if the colonies survive exposure and are resistant to any or all antibiotics.